Those of us who have worked in broadcasting know that most AM
broadcasters use a heavy copper mesh (typically 40' - 64' square)
surrounding the base of a tower, with radials connected at the
perimeter. I recall KM1H saying that he's used a mesh for years, and
I've recently come across NC0B's discussions on the topic. He first
wrote about it in Ham Radio in May 1977, and did a presentation at
Dayton in 2009.
He describes an installation of a 60 ft Tee vertical with 200 ft
horizontal and 600 sq ft of mesh laid out in strips at 90 degrees to
each other, centered at the feedpoint, then with the addition of 20 or
40 radials 100 ft long.
The gist of his work is that 400 - 600 sq ft of mesh is a good design
goal for antennas of that height, that 1/2-inch galvanized hardware
cloth works.Measurements of the 60 ft Tee vertical with the ground
screen alone was 4 dB below an ideal ground, adding 20 or 40 radials 100
ft long increased field strength by 0.4 and 0.9dB respectively.
When thinking about this, bear in mind that the radius illuminated by
the fields from a vertical are directly related to the height of the
vertical, so a taller antenna would require longer strips of mesh to
achieve comparably high values of efficiency. But in his Ham Radio
article describes some quite small ground screens that had to fit in
very limited spaces, and says "that they worked well." On the other
hand, his 2009 slides consider 100 sq ft a minimum.
Both the paper and the presentation are well worth study. The Ham Radio
article is on his own website, the Dayton slides are on K3LR's website.
73, Jim K9YC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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